1. Field of the Invention
The present invention relates to a transmission device, and in particular to a transmission device, provided with a radio frequency or high frequency (hereinafter, represented by RF) power amplifier, applied to a radio base station.
2. Description of the Related Art
An RF power amplifier (hereinafter, abbreviated as amp) in a transmission device applied to a radio base station has the following functions:                Function capable of switching ON or OFF a source of the power amp;        Function capable of monitoring an input level of the power amp;        Function capable of monitoring an output level of the power amp.        
A prior art arrangement of such a transmission device is shown in FIG. 10. In this transmission device, when having received data X (I, Q) of a base band as an input, a distortion compensator 1 multiplies to the input data a distortion compensating coefficient of an RF power amp 6. An output signal of the distortion compensator 1 applied with the distortion compensating coefficient is converted by a D/A converter 2 into an analog IQ signal to be provided to a modulator 3. This modulator 3 performs a direct modulation with a local oscillator 4 of a carrier frequency (for example, 2.1 GHz bandwidth) to generate an RF signal.
The RF signal thus generated passes through a coupler 5 that forms an input signal detector of an RF power amp 6 and inputted to the amp 6. A signal power-amplified by the RF power amp 6 passes through a coupler 7 that forms an output signal detector of the RF power amp 6 together with a coupler 13 and transmitted to an antenna 8. It is to be noted that the couplers 5 and 7 respectively branch their input signals to an amp input monitor terminal 11a and an amp output monitor terminal 11b, and that a source of the RF power amp 6 is switched ON or OFF by a power amp switch 10.
For updating the distortion compensating coefficient of the distortion compensator 1, the coupler 13 provides a branched signal from its input signal to a frequency converter 15, which downconverts its input signal into an intermediate frequency (IF) signal with a local oscillator 16. The output signal of the frequency converter 15 is converted into a digital signal by an A/D converter 17 and provided to the distortion compensator 1, forming a feedback route. The distortion compensator 1 compares the input data X (I, Q) with the feedback data from the A/D converter 17 to update the distortion compensating coefficient, thereby suppressing distortion components of the RF power amp 6.
When a switching signal KS from the power amp switch 10 is made OFF or reset, the source within the RF power amp 6 is powered down so that its transmission output is suspended. For example, when experiments are desired to be performed while suspending the transmission output of the power amp 6, the power amp switch 10 is made OFF, whereby in the absence of a transmission wave from the antenna 8, such an event that the RF power amp 6 is faulted due to a reflecting wave does not occur even though the output of the antenna 8 is not terminated.
The amp input monitor terminal 11a outputs the input signal of the RF power amp 6 which is branched at the coupler 5, which is used in such a case where the transmission wave is required to be confirmed at the above experiments. Even at the suppression of the RF power amp 6, a signal comes in up to the input terminal of the RF power amp 6, so that the transmission wave can be recognized by monitoring the signal.
The amp output monitor terminal 11b provides the output signal of the RF power amp 6 coupled by e.g. several 10 dB at the coupler 7. This enables the transmission output waveform to be recognized at the output terminal of the RF power amp 6 even upon the antenna 8 being connected, and also the transmission output level to be recognized if a coupling quantity of the coupler 7 is prescribed. It is of course that the transmission waveform can not be observed when the power amp switch 10 is made OFF.
As a reference document, there has been proposed a plural-mode shared transmitting circuit comprising local oscillation circuits, quadrature modulating circuits, gain variable circuits, power amp circuits and output monitor circuits respectively provided for each mode, in which a switch which selects monitor signals outputted from the output monitor circuits of the respective modes and inputs them into an output wave detecting circuit is as to enable sharing the output wave detecting circuit and a difference detecting circuit by the plural modes and the switch which selects differential voltage outputted from the difference detecting circuit and inputs them into gain variable circuits are provided; thus, an automatic power control circuit is shared so that the circuit scale and volume are reduced and the cost is also reduced (See e.g. Patent document 1).                [Patent document 1] Japanese Patent Application Laid-open No. 9-312578(1) Problem on Mounting        
While downsizing the radio base station is a big issue for recent customers' demands, the above prior art is disadvantageous in that it has two monitor terminals, i.e. an amp input monitor terminal and an amp output monitor terminal, causing the surface area of the device to be enlarged.
(2) Problem on Operability
The above prior art uses the amp input monitor terminal at the time when the source of the power amp is turned or switched OFF, while using the amp output monitor terminal at the time when the source of the power amp is switched ON. Therefore, each time the power amp is switched ON or OFF, the connections of the monitor terminals have to be changed, resulting in a poor operability.
(3) Problem on Distortion Compensation
As shown in FIG. 11, a predistortion type distortion compensation is a technology for decreasing distortions at the output terminal of the power amp by predicting a distortion quantity f(p) based on a non-linear characteristic of the power amp and applying a signal preliminarily multiplied with a reverse distortion characteristic h(p) of the power amp to the input signal x(p) (step S100) (step S200). Resultantly, at the amp input monitor terminal, a signal h(p)*x(p) applied with the reverse distortion characteristic h(p) of the power amp is to be observed.